The transition to a world with an oxygenated deep ocean occurred between 540 and 420 million years ago, new research suggests.

Researchers attribute the change to an increase in atmospheric O2 to levels comparable to the 21 percent oxygen in the atmosphere today.

This inferred rise comes hundreds of millions of years after the origination of animals, which occurred between 700 and 800 million years ago.

By measuring the oxidation of iron in pillow basalts from undersea volcanic eruptions, scientists have more precisely dated the oxygenation of the deep ocean, inferring from that when oxygen levels in the atmosphere rose to current high levels. (Credit: UC Berkeley)

“The oxygenation of the deep ocean and our interpretation of this as the result of a rise in atmospheric O2 was a pretty late event in the context of Earth history,” says Daniel Stolper, an assistant professor of Earth and planetary science at the University of California, Berkeley.

“This is significant because it provides new evidence that the origination of early animals, which required O2 for their metabolisms, may have gone on in a world with an atmosphere that had relatively low oxygen levels compared to today.”

Tracing oxygen’s history

Oxygen has played a key role in the history of Earth, not only because of its importance for organisms that breathe it, but because of its tendency to react, often violently, with other compounds to make iron rust, plants burn, and natural gas explode.